One of the major obstacles to curative chemotherapy is multidrug resistance (MDR), innate or acquired. Resistance to doxorubicin (DX) may be associated with overexpression of membrane p-glycoprotein (PGP-170), and /or other mechanisms, and may involve multiple drug resistances (MDR). Reversal of MDR has been studied mostly with highly resistant cells in vitro, and little is known about cellular heterogeneity in MDR, or cellular heterogeneity in reversal of MDR. The goal of this project is to identify factors associated with the in vitro resistance to DX in subsets of tumor cells isolated from populations with graded degrees of resistance, to develop approaches for their modulation. Knowledge gained from in vitro results will be applied in vivo to evaluate the possibility of overcoming resistance therapeutically. To this end, mouse and human cell lines. (P388, KB HeLa and ovarian carcinoma A2780), and sublines of graded degrees of resistance to adriamycin, have been characterized in vitro and established to grow in vivo. Although P388, KB and A2780 cells express PGP-170 in graded degrees, HeLa cells provide the model for multidrug resistance without overexpression of PGP-170. To isolate cells with a specific mechanism of resistance, e.g. overexpression of PGP-170 and decreased DX accumulation, flow cytometry, FACSTAR, will be used; cell subpopulations will be assessed for individual colony formation (iCFA) cellular heterogeneity in growth and DX sensitivity with and without the modulators, DMDP, a new calcium channel blocker, and dipyridamol, a nucleoside transport inhibitor will be the modulators to be studied. Unlike verapamil, effective in vitro concentrations of DMDP could be achieved in vivo without host toxicity. The specific aims of this proposal are: to identify determinants of response and resistance to DX in whole cells populations and in subsets with graded degrees of MDR; 2) to define the schedule and effective noncytotoxic doses of the modulator, dipyridamol and a calcium channel blocker, DMDP, required to reverse graded levels and mechanisms of resistance and (3) to establish in vivo model systems mimicking the conditions found optimal in vitro, in order to evaluate the diversity in therapeutic response and selectivity of DX in combination with the modulator against cells resistant to DX. These data should provide a basis for the development of specific and more selective treatments of tumor exhibiting multidrug resistance characteristics.